THE constant increase in power electronic devices, used

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1 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 8, AUGUST Cascaded Nine-Level Inverter for Hybrid-Series Active Power Filter, Using Industrial Controller Alexander Varschavsky, Juan Dixon, Senior Member, IEEE, Mauricio Rotella, and Luis Morán, Fellow, IEEE Abstract An industrial controller, specifically designed for two- and three-level converters, was adapted to work on an asymmetrical nine-level active power filter (APF). The controller is now able to make all required tasks for the correct operation of the APF, such as current-harmonic elimination and removal of high-frequency noise. The low switching-frequency operation of the nine-level converter was an important advantage in the application of the industrial controller. In addition, with the nine-level filter, switching losses were significantly reduced. The filter was designed to work as voltage source and operates as harmonic isolator, improving the filtering characteristics of the passive filter. The control strategy for detecting current harmonics is based on the p q theory and the phase-tracking system in a synchronous reference frame phase-locked loop. The dc-link voltage control is analyzed together with the effect of controller gain and delay time in the system s stability. Simulations for this application are displayed and experiments in a 1-kVA prototype, using the aforementioned industrial controller, were tested, validating the effectiveness of this new application. Index Terms Active filters, harmonic distortion, multilevel systems, power quality. I. INTRODUCTION THE constant increase in power electronic devices, used by industrial and commercial consumers, has deteriorated seriously electric power systems. More transmission losses, power-transformer and neutral-conductor overheating, powerfactor correction-capacitor overloading, and induced noise in control systems are only a few of the problems that harmonic distortion may bring into home and industrial installations [1], generating considerable economic losses to distribution companies and end users []. During many years, the solution used to minimize harmonic pollution has been tuned passive filters. However, they have quite a few disadvantages, like fixed compensating characteristic (given only by the tuned frequencies), parallel and series resonance with source-voltage harmonics, and filtering Manuscript received October, 008; revised August 19, 009; accepted September 6, 009. Date of publication October 0, 009; date of current version July 14, 010. This work was supported by the Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) through Project Fondecyt , ABB Chile, and Millenium Project P F. A. Varschavsky is with the CGE Distribución S.A., Santiago , Chile ( auvarsch@uc.cl). J. Dixon is with the Department of Electrical Engineering, Pontificia Universidad Catolica de Chile, Santiago , Chile ( jdixon@ing.puc.cl). M. Rotella is with the ABB S.A., Santiago , Chile ( mauricio. rotella@cl.abb.com). L. Morán is with the Department of Electrical Engineering, Universidad de Concepción, Concepción 53-C, Chile ( luis.moran@udec.cl). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TIE characteristics strongly affected by source impedance. They are also bulky, and they lost their effectiveness with the passage of time [3], [4]. Several topologies of active power filters (APFs) have been proposed [5] [7] as a solution to passive-filter problems. Most of the APFs that have been implemented until now are of shunt type [8] [11], but they are comparatively more expensive due to its large rating of about 30% 60% of the load [1]. Even more, they cannot compensate correctly for harmonic voltage produced by power rectifiers with large dc-link capacitor [13]. Numerous series APF have been proposed [3], [4], [1], [14], [15], most of them operating as hybrid, in conjunction with shunt passive filters. The advantages of hybrid topologies are quite significant since the series active filter can be very low rated, between 3% and 10% [1], and the disadvantages of the passive filters are mitigated. Another advantage of this hybrid topology is that harmonic voltage and current-producing loads can be effectively compensated. However, series active filters proposed until now had been implemented in two-level PWM based inverters, with the known disadvantages that they present, such as high-order harmonic noise and additional switching losses due to high-frequency commutation [16]. Multilevel inverters have become very popular in the last few years, due to their capability to generate cleaner voltage waves and lower switching losses [17], [18]. If the cascaded H-bridge topology scaled in powers of three is utilized, the number of sources and semiconductors is minimized [19] [4]. With this topology, each H bridge operates at a lower frequency, decreasing switching losses and permitting the use of slower semiconductors. This paper shows that lower frequency operation of the asymmetrical converter has permitted the adaptation of industrial controllers for filtering purposes. II. SYSTEM DESCRIPTION A. System Configuration The circuit of Fig. 1 shows the basic topology of the system, which is composed by three 9-level inverters connected in series between the source and the load and a shunt passive filter tuned at fifth and seventh harmonics. The passive filter presents a low-impedance path to load-current harmonics and also helps to partially correct the power factor. B. Multilevel Inverter Each phase of the nine-level series APF comprises two H bridges connected at the same dc-link capacitor. The two bridges are connected to the ac line using independent /$ IEEE

2 76 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 8, AUGUST 010 Fig. 3. Single-phase equivalent circuit for harmonics. Fig. 1. Main components of the hybrid-series APF. equal to the harmonic voltage appearing across the resistor. The load is represented as a harmonic voltage source. Equation () can be directly derived from Fig. 3 and (1) i s,h v s,h v L,h Z s + K. () If K is large enough (several times Z s ), the harmonic current i s,h becomes zero, and an almost purely sinusoidal current is drawn from the utility. The voltage at the point of common coupling v PCC is also improved with the action of the filter, depending only on the utility-voltage distortion instead of the harmonic voltage of the load. Fig.. Multilevel inverter topology (one phase). transformers scaled in power of three, as shown in Fig.. More than nine levels increases hardware complexity and does not improve noticeably the filtering characteristics. The H-bridge converter is able to produce three levels of voltage at the ac side: +v dc, v dc, and zero. The outputs of the modules are connected through transformers whose voltage ratios are scaled in power of three, allowing 3 n levels of voltage [3]. Then, with only two converters per phase (n ),nine different levels of voltage are obtained: four levels of positive values, four levels of negative values, and zero. The transformer located at the bottom of the figure has the highest voltage ratio and, with its corresponding H bridge, is called main converter. The other transformer defines the auxiliary converter (Aux). The main converter manages most of the power but works at the lowest switching frequency, which is an additional advantage of this topology. Amplitude modulation is used to determine the output level of the inverter, rounding the reference signal to the nearest integer between the nine possible levels. C. Operation Principle In order to eliminate line-current harmonics, the APF is controlled to present very high impedance to these currents. This is achieved by generating a voltage proportional to the harmonic current i s,h. To keep the dc-link voltage constant, it is necessary to add to the reference signal a compensating signal of fundamental frequency, being the reference voltage to each phase as shown in v AF K i s,h + v AF,f. (1) Fig. 3 shows an equivalent circuit for the harmonics, where the APF is represented as a resistor of value K since v AF,h is D. Harmonic-Current Detection To detect the harmonic content of the line current, the socalled p q theory [5] was used. According to this theory, when three symmetrical and sinusoidal source voltages and three asymmetrical or distorted line currents are present in a three-phase power system, the dc components i p_dc and i q_dc of the instantaneous active and reactive currents i p and i q correspond to the fundamental active and reactive components of the line current, whereas the corresponding ac components of i p and i q, i p_ac and i q_ac, are related to the asymmetric and harmonic components. The complete control scheme of the APF is shown in Fig. 4. In the diagram, it can be seen that the instantaneous active and reactive currents i p and i q are obtained by transforming the instantaneous source currents i a, i b, and i c (which is obtained by subtracting both i a and i b ) by means of Clark and Park transformations as shown in [ ] [ ] [ ip sin ωt cos ωt ] i q 3 cos ωt sin ωt i a i b. i c (3) A low-pass filter (LFP) then extracts the dc components of i p and i q, resulting in i p_dc and i q_dc. A compensating dc-link voltage is subtracted from the fundamental active current i p, and the resulting signal, together with i q_dc are transformed back to a three-phase system by the inverse Park and Clark transformations as shown in (4). The cutoff frequency of the LPF used in this paper was 10 Hz i a 1 0 i b i c 1 3 [ sin ωt cos ωt cos ωt sin ωt ] [ ip i q ]. (4)

3 VARSCHAVSKY et al.: CASCADED NINE-LEVEL INVERTER FOR HYBRID-SERIES ACTIVE POWER FILTER 763 Fig. 4. p q-based control-system scheme of the APF. Fig. 5. DC-link voltage-control block diagram. Then, this three-phase reference signals, which correspond to the fundamental active and reactive components, are subtracted from the line currents; In this way, the harmonic-current component plus the dc-link compensating voltage are obtained. A K factor is then multiplied to the output signal of the system so that the APF acts as a high-harmonic impedance. To get the reference signals sin and cos shown in Fig. 4, a synchronous reference frame phase-locked loop (PLL) was used [6]. A low-bandwidth proportional integral (PI) controller was used in the PLL control loop so that the reference signals stay clear and in phase with the source, even under the presence of source harmonics. E. DC-Link Voltage Control The capacitor voltage at the dc-link must remain constant so that the APF can work properly. This voltage should not fluctuate because the APF should neither supply nor absorb active power from the utility. However, due to commutation, conductance, and own-capacitor losses plus control system delays, dc-link voltage fluctuations occur. To compensate the voltage variations, the filter operates as a controlled rectifier, transferring a little amount of active power from the ac to the dc side of the inverter. This is achieved by generating a sinusoidal voltage signal in phase with the line current, whose product is the transferred active power. Owing to the fact that the three inverters share the dc capacitor, only one control system is needed, which output signal is added to the fundamental active component of the line current i p_dc obtained as was shown in Fig. 4. Fig. 5 shows the block diagram of a dc-link control loop, where C(s) is the transfer function of the PI controller and G(s) the transfer function between the capacitor voltage v dc and the compensation current generated by the PI controller i e. The transfer function G(s) can be derived directly doing a power balance between the ac and dc side of the three inverters. This yields to 3 K ie (t) I s cos φ C v dc (t) d dt v dc(t) (5) where I s is the effective value of line current. Doing a linearization of (5) at the operation point v dc (t) v dc_o and i e (t) i e_o, where v dc_o is the reference voltage from the capacitor, the transfer function G(s) can be obtained as follows: G(s) Δv dc 3 K Is cos(φ). (6) Δi e s C v dc_0 Since the controller is a proportional plus integral one, its transfer function is ( C(s) K P 1+ 1 ). (7) T I s Then, the closed-loop transfer function of the dc-link control system is as shown in Δv dc Δv ref s + Cv dc_0 Cv dc_0 s + s + T I Cv dc_0 T I Cv dc_0. (8) Equation (8) corresponds to a second-order system, where the damping factor ζ and natural angular velocity ω n can be calculated from (9) and (10). Since the poles of the system are always at the left side of the s-plane, the system is always stable 3 ζ 1 KP T I K I s cos(φ) (9) C v dc_0 3 KP K I s cos(φ) ω n. (10) T I C v dc_0 For simulations and experiments, ζ 1 and ω n π 100 rad/s were used. In this way, a critically damped response, where minimal voltage oscillation happens, is obtained. F. Stability Analysis for Harmonic Currents The delay times introduced by the controller, large gain K, and high system stiffness seriously affect the active-filter stability [7]. Assuming purely inductive source impedance, (11) can be directly derived from Fig. 3 i s,h v s,h v L,h v AF,h s L s. (11)

4 764 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 8, AUGUST 010 TABLE I PASSIVE FILTERS USED Fig. 6. Harmonic-current-control diagram. Fig. 7. Series APF prototype used in the experiments. If a delay time τ is introduced by the controller, the harmonic voltage generated by the filter in Laplace domain is v AF,h K i s,h e sτ. (1) Hence, the closed-loop block diagram of the harmonic control system can be represented as shown in Fig. 6. From the Nyquist stability criterion, the selected gain K of the filter must satisfy (13) so that the system will be stable K< π τ L s. (13) For a typical line inductance L s 0.0 per unit (p.u.) in a 50-Hz system and a gain K, the delay time should be less than 50 μs, which will ensure system stability. III. SIMULATIONS AND EXPERIMENTAL RESULTS The proposed hybrid-series APF was simulated in MATLAB/Simulink, using a source impedance of % p.u. For the experiments, a 1-kVA/110-V 50-Hz system using an IGBT-based multilevel inverter was implemented. All the control tasks in the experiments were performed by the aforementioned industrial controller, which combines a 600-MHz CPU and a large field-programmable gate array (FPGA). The controller and the complete experimental system are shown in Fig. 7. The high-speed control tasks are programmed into the industrial controller using the MATLAB/Simulink. This allows implementing the same model used in simulations for real-world operation, ensuring an error-free and fast implementation. The complete control loop is executed by the controller every 50 μs. Fig. 8. Simulated transient response under APF connection. (a) Source voltage v s. (b) Line current i s. (c) Load current i L. (d) Active-filter voltage v AF. (e) Capacitor voltage v dc. A gain K 4, equivalent to p.u. was used in both simulation and experiments. The turn ratios of the output transformers were 0 : 18 and 0 : 54 for the auxiliary and main transformers, respectively. The dc-link capacitor was selected to be C 4700 μf. The load connected to the six-pulse diode rectifier was R L Ω, and C L 4700 μf. The values of the shunt passive filters can be seen in Table I. The quality factor Q of the passive filters is very low in comparison with industrial passive filters that operate without an active section. This can be an advantage considering that the cost of passive filters with a lower Q factor is lower. The passive filters were also slightly off tuned, decreasing its performance even more. In the simulations shown in Fig. 8, the active filter starts to operate at t 0.08 s. It can be seen that the source voltage and line current are improved with a negligible delay, changing to an almost purely sine wave. As the load current i L does not change with APF operation, the rectifier operation is not being affected. It is quite important to mention that the maximum switching frequency of the main converter (which manages 70% of the kilovolt-ampere) is only 500 Hz. This small switching frequency allows using the APF in the high-power range, using the relatively slow gate-turn-off thyristor. On the other hand, the Aux, which only manages 30% of the power,

5 VARSCHAVSKY et al.: CASCADED NINE-LEVEL INVERTER FOR HYBRID-SERIES ACTIVE POWER FILTER 765 Fig. 9. Line-current and source-voltage harmonic contents (a) without filters, (b) with passive filters, and (c) with active and passive filters. switches at a maximum frequency of only khz. For a very high power application using two-level converters and PWM to compensate, the switching frequency must be at least 10 khz to get a similar result. Following the analysis given in Fig. 8, a considerable fundamental voltage appears at the terminals of the active filter, which is due to resistive losses at the ac side of the filter transformers. This problem can be improved using better quality transformers. The capacitor voltage v dc follows its reference, and minimal fluctuations appear when the filters start their operation. The harmonic content and total harmonic distortion (THD) of line current and source voltage without filters, only with passive filters and with both passive and active sections, are shown in Fig. 9. It can be seen that the line-current THD dramatically improves with the APF, passing from 0% to.4%, and hence, all harmonics goes to almost zero. The THD of the line current without passive filters was 34%. The sourcevoltage THD improves from.3% to 0.65% when the APF starts its operation. The improvement in this case depends on utility stiffness; if the system is very strong, the source voltage is not too affected by harmonic currents flowing to the load, and the action of the APF cannot be seen clearly. In the other case, if the system is weak, line current affects the source voltage noticeably, and the effect of the APF is more evident. In Fig. 10, the response of the active filter under a load change can be seen. The load of the rectifier changes from RL Ωto RL 44Ω, without affecting neither the sourcecurrent clearness nor dc voltage stability. Experimental results corroborate the successful operation of the active filter shown in the simulations. Fig. 11 shows the transient response when APF starts to operate. The line current immediately becomes sinusoidal and the capacitor-voltage behavior is as expected. The reduction in harmonic content, as seen on Fig. 1, is also similar to the simulated one, but in the experiments, the effect of passive filters is greater, the line-current THD, with only the passive filters connected, being 15%. This is due to bigger line inductance introduced by the autotransformer used to connect the system to the grid. Fig. 10. Simulated transient response after load change. (a) Source voltage v s. (b) Line current i s. (c) Load current i L. (d) Active-filter voltage v AF. (e) Capacitor voltage. v dc. Fig. 11. Experimental transient response under APF connection. From top to bottom: Active-filter voltage v AF, line current i s, and capacitor voltage v dc. Fig. 1. Left: line-current-harmonic content without APF. Right: line-currentharmonic content with APF operating.

6 766 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 8, AUGUST 010 capacitor voltage returns to its reference in two cycles, with minimum variation when the load changes. In the experimental results, the load was increased from R L 44Ωto R L Ω. IV. CONCLUSION A hybrid-series AFP based on a low-rated multilevel inverter, acting as a high-harmonic impedance, and a shunt passive filter acting as a harmonic-current path, were developed and tested. All the control tasks were programmed in an industrial controller, which was adapted for this particular application. With the proposed control algorithm and taking advantage of the multilevel topology of the active filter, almost purely sinusoidal currents and voltages were achieved, without the usual highfrequency content present in PWM inverters The proposed filter, which compensate harmonic-voltage source generated by contaminating loads, was simulated and tested using MATLAB/Simulink. It has been demonstrated that the filter responds very fast under connections and sudden changes in the load conditions, reaching its steady state in about two cycles of the fundamental. Fig. 13. Line current and active-filter voltage close-up. Up: Only with passive filter. Bottom: With both active and passive filters operating. Fig. 14. Experimental transient response under load change. From top to bottom: Active filter voltage v AF, line current i s, and capacitor voltage v dc. A detailed waveform of the line current and active-filter voltage, without and with the APF, can be seen in Fig. 13, where it becomes evident the effect of the APF. In addition, no highfrequency commutation content, like the ones introduced by PWM-based converters, is present. The importance of this feature is that this kind of filter behaves almost like an ideal APF. In Fig. 14, the response of the series active filter under load change is shown. The line current remains sinusoidal and the REFERENCES [1] W. Mack Grady and S. Santoso, Understanding power system harmonics, IEEE Power Eng. Rev., vol. 1, no. 11, pp. 8 11, Nov [] H. Rudnick, J. Dixon, and L. Moran, Delivering clean and pure power, IEEE Power Energy Mag., vol. 1, no. 5, pp. 3 40, Sep./Oct [3] F. Z. Peng, H. Akagi, and A. Nabae, A new approach to harmonic compensation in power systems A combined system of shunt passive and series active filters, IEEE Trans. Ind. Appl., vol. 6, no. 6, pp , Nov./Dec [4] Z. Wang, Q. Wang, W. Yao, and J. Liu, A series active power filter adopting hybrid control approach, IEEE Trans. Power Electron., vol. 16, no. 3, pp , May 001. [5] B. Singh, K. Al-Haddad, and A. Chandra, A review of active filters for power quality improvement, IEEE Trans. Ind. Electron., vol. 46, no. 5, pp , Oct [6] F. Z. Peng, Harmonic sources and filtering approaches, IEEE Ind. Appl. Mag., vol. 7, no. 4, pp. 18 5, Jul./Aug [7] H. Akagi, Modern active filters and traditional passive filters, Bull. Pol. Acad. Sci. Tech. Sci., vol. 54, no. 3, pp , Sep [8] B. Singh and J. Solanki, Implementation of an adaptive control algorithm for a three-phase shunt active filter, IEEE Trans. Ind. Electron., vol. 56, no. 8, pp , Aug [9] B. Kedjar and K. Al-Haddad, DSP-based implementation of an LQR with integral action for a three-phase three-wire shunt active power filter, IEEE Trans. Ind. Electron., vol. 56, no. 8, pp , Aug [10] F. D. Freijedo, J. Doval-Gandoy, O. Lopez, P. Fernandez-Comesana, and C. A. Martinez-Penalver, Signal-processing adaptive algorithm for selective current harmonic cancellation in active power filters, IEEE Trans. Ind. Electron., vol. 56, no. 8, pp , Aug [11] E. Lavopa, P. Zanchetta, M. Sumner, and F. Cupertino, Real-time estimation of fundamental frequency and harmonics for active shunt power filters in aircraft electrical systems, IEEE Trans. Ind. Electron., vol. 56, no. 8, pp , Aug [1] J. W. Dixon, G. Venegas, and L. A. Moran, A series active power filter based on a sinusoidal current-controlled voltage-source inverter, IEEE Trans. Ind. Electron., vol. 44, no. 5, pp , Oct [13] F. Z. Peng, Application issues of active power filters, IEEE Ind. Appl. Mag., vol. 4, no. 5, pp. 1 30, Sep./Oct [14] S. Bhattacharya and D. Divan, Design and implementation of a hybrid series active filter system, in Proc. Power Electron. Spec. Conf., 1995, vol. 1, pp [15] A. Hamadi, S. Rahmani, and K. Al-Haddad, A novel hybrid series active filter for power quality compensation, in Proc. Power Electron. Spec. Conf., 007, pp [16] J. Dixon and L. Moran, Multilevel inverter, based on multi-stage connection of three-level converters scaled in power of three, in Proc. IEEE IECON, 00, vol., pp

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Power Electron., vol. 17, no. 1, pp , Jan. 00. Juan Dixon (M 90 SM 95) was born in Santiago, Chile. He received the B.S. degree in electrical engineering from the Universidad de Chile, Santiago, Chile, in 1977 and the M.S.Eng. and Ph.D. degrees from McGill University, Montreal, QC, Canada, in 1986 and 1988, respectively. In 1976, he was with the State Transportation Company in charge of trolleybuses operation. In 1977 and 1978, he was with the Chilean Railways Company. Since 1979, he has been with the Electrical Engineering Department, Pontificia Universidad Catolica de Chile, Santiago, where he is currently a Professor. He has presented more than 70 works in international conferences and has published more than 30 papers related with power electronics in IEEE Transactions and IEE proceedings. His main areas of interests are in electric traction, power converters, PWM rectifiers, active power filters, power-factor compensators, multilevel, and multistage converters. He has consulting work related with trolleybuses, traction substations, machine drives, hybrid electric vehicles, and electric railways. He has created an electric vehicle laboratory where he has built state-of-the-art vehicles using brushless dc machines with ultracapacitors and high specific-energy batteries. Mauricio Rotella received the Electrical Engineer Professional and Master of Science degrees from Pontificia Universidad Católica de Chile, Santiago, Chile, in 1999 and 006, respectively. Since 1998, he has been working with ABB in various positions, including product manager for large drives in ABB Chile and Regional Sales Manager for Latin America MV Drives in ABB Switzerland. Currently, he is with Automation Products Local Division in ABB Chile. Alexander Varschavsky was born in Santiago, Chile. He received the B.S. degree in electrical engineering and the M.Sc. degree from the Pontificia Universidad Católica de Chile, Santiago, in 008. From 007 to 008, he was a Student Researcher with the Núcleo de Electrónica Industrial y Mecatrónica, Santiago, where he was involved with the design and application of power electronics devices. Currently, he is an Engineer with CGE Distribución, Santiago, a Chilean utility company. His research interests are active power filters, electrical machines, power electronics, and power systems. Luis Morán (F 05) was born in Concepción, Chile. He received the Ph.D. degree from Concordia University, Montreal, QC, Canada, in Since 1990, he has been with the Electrical Engineering Department, University of Concepción, Concepción, where he is a Professor. He has written and published more than 30 papers in active power filters and static Var compensators in IEEE Transactions. His main areas of interests are in ac drives, power quality, active power filters, FACTS, and power protection systems. Dr. Morán is the principal author of the paper that got the IEEE Outstanding Paper Award from the Industrial Electronics Society for the best paper publishedintheieeetransaction ON INDUSTRIAL ELECTRONICS during 1995, and the coauthor of the paper that was awarded in 00 by the IAS Static Power Converter Committee.